QToolButton with popupMode QToolButton::MenuButtonPopup has inaccessible menu when defaultAction disabled

The menu of a QToolButton with with popupMode QToolButton::MenuButtonPopup becomes inaccessible when its defaultAction is disabled. Instead only the button part but not the menu part should become disabled. A common function the popup menu is used for is to subsume toolbar functions under the menu to leave more space for other buttons on the toolbar. This is standard behaviour in other common application, e.g. MS Word.